The present invention relates to transducer head assemblies for rotating disc drives, and more particularly to self-loading air bearing sliders.
Transducer head assemblies that "fly" relative to a rotating disc are used extensively in rotating disc drives. A head assembly includes a gimbal and an air bearing slider for carrying a magnetic transducer proximate the rotating disc. An actuator arm positions the slider and the transducer over individual data tracks on the disc surface. The gimbal is positioned between the slider and the actuator arm to provide a resilient connection which allows the slider to follow the topography of the disc. The gimbal includes a dimple that is in point contact with the slider. The dimple provides a point about which the slider can pitch and roll while following the topography of the disc.
The slider includes a pair of side rails which are positioned along its side edges and are disposed about a recessed area. The side rails form a pair of air bearing surfaces. As the disc rotates, the disc drags air under the slider and along the air bearing surfaces in a direction approximately parallel to the tangential velocity of the disc. As the air passes beneath the side rails, skin friction on the air bearing surfaces causes air pressure between the disc and the air bearing surfaces to increase which creates a hydrodynamic lifting force that causes the slider to lift and fly above the disc surface.
A self-loading, negative (or subambient) pressure air bearing slider (NPAB) includes a cross rail which extends between the side rails and is positioned near the slider's leading edge. The cross rail can also be referred to as a "throat" or a "dam". The cross rail forms a subambient pressure cavity trailing the cross rail, between the side rails. The subambient pressure cavity is typically five to ten microns deep. The air passing beneath the slider expands in the cavity, resulting in a decrease in pressure. The pressure in the cavity may become subambient, in which case the integral of the pressure over the cavity area provides a self-loading force on the slider which forces the slider toward the disc surface. The subambient pressure or suction that is developed in the cavity is a function of cross-bar or cross-rail height above the disc divided by the depth of the cavity. The ratio of cross-rail height to cavity depth is the expansion ratio of the air crossing the cross-rail. The self-loading force counteracts the hydrodynamic lifting force developed along the side rails. The counteraction between positive and negative forces on the slider reduces flying height sensitivity with respect to disc velocity and increases air bearing stiffness.
The disc tangential velocity is greater at an outer disc diameter than at an inner disc diameter. The magnitude of the positive pressure developed along the side rails increases with the sliding velocity. However, the magnitude of the self-loading force also increases with the sliding velocity. The increasing self-loading force prevents the increasing positive pressure from forcing the slider away from the disc. The equilibrium clearance of the self-loading air bearing slider is therefore less dependent on sliding velocity than a conventional air bearing slider.
The self-loading air bearing slider is also stiffer than the conventional air bearing slider. This effect is due to relatively large surface areas that are required to support the slider at a specified clearance. The surface area of the self-loading bearing must be larger than that of a conventional bearing, to provide adequate lifting force to resist the self-loading force as well as a spring pre-load force applied by the actuator arm.
It has been found that the advantages of the self-loading air bearing slider are maximized by making the subambient pressure cavity area as large at possible. Warner et al., U.S. Pat. No. 4,475,135, disclose a self-loading air bearing slider having a pair of side rails and a cross rail which is positioned at the slider's leading edge. The cross rail lies in a plane defined by the side rails and includes a full-width taper at the leading edge. The full-width taper provides a faster liftoff from the disc surface.
Although the slider disclosed by Warner et al. maximizes the area of the subambient pressure cavity, it also has undesirable features. First, the full-width leading taper tends to collect wear particles and similar debris. This debris sheds occasionally and is dragged between the slider and the disc, causing increased wear to the air bearing surfaces and the disc surface. Second, the cross rail and the leading edge taper cause the slider to fly with an unusually high pitch angle. A very high pitch angle degrades the stiffness of the air bearing.
Chapin et al., U.S. Pat. No. 5,210,666, disclose a self-loading air bearing slider with a relieved leading edge. The cross rail includes a relief or "notch" which is recessed from the air bearing surfaces. The "notch" minimizes debris collection at the leading edge and reduces pitch angle. Various other flying characteristics are also improved, such as reduced flying height sensitivity to altitude and higher vertical and roll stiffness.
However, the slider disclosed by Chapin et al. requires several fabrication steps to manufacture. When fabricated by an ion milling process, the slider is initially lapped to a smooth and flat surface suitable for application of milling pattern masks. To fabricate the leading taper at the leading edge, the leading edge of the slider is lapped at an angle. Next, two etching cycles using two separate masks are required to create the desired air bearing topography. One etching cycle is required to recess the cross rail from the air bearing surface by a depth of about 1.2 micron. Another etching cycle is required to form the relatively deep subambient pressure cavity. The material left between the pattern masks defines the cross-rail. Because residual lapping stresses are relieved by the etching, some warpage of the slider typically occurs. As a result, an additional lapping step is performed on a spherically dished lapping plate to obtain a flying surface with a specified crown height. The reflat process removes approximately 0.2 microns such that the cross rail has a finished cross rail depth of about 1.0 microns below the air bearing surface. The additional lapping step (known as a reflat process) assures that warping is removed and that the slider is properly crowned.
Similarly, Strom et al., U.S. Pat. No. 5,062,017, disclose a self-loading hour-glass disc head slider that requires at least a two cycle etching process for fabrication. One etching cycle is required to create relatively shallow, partial edge steps into a pair of raised side rails to form generally hour-glass shaped air bearing surfaces. The cross rail is etched at the same time as the partial edge steps to relieve the cross rail from the air bearing surfaces by the same depth as the partial edge steps. Another etching cycle is required to form a relatively deep subambient pressure cavity between the pair of raised side rails. The material left between the two patterned masks used in the two etching cycles defines the cross-rail. As with the slider disclosed by Chapin et al., the slider disclosed by Strom et al. experiences warpage caused by residual lapping stresses being relieved by the etching cycles. Consequently, the reflat process is also performed on the slider of Strom et al. to obtain a flying surface with a specified crown height.
Each of the fabrication steps required to manufacture the sliders disclosed by Chapin et al. and Strom et al. cause variations in the finished slider cross rail height above the disc surface. Because each of the fabrication steps are not entirely controllable, variations in the initial 1.2 micron etching fabrication step and the reflat stock removal cause variations in the height of the cross rail. Because the cross rail height determines the suction that is developed in the cavity, the cross rail height significantly effects flying height of the finished slider. Variations in the cross rail height cause variations of subambient pressures within the pressure cavity which in turn creates flying height variation.
Moreover, each additional fabrication step that is required to manufacture advanced air bearing sliders increases the time required for fabrication, increases slider fabrication cost, reduces slider yield by stacking up additional tolerances, and complicates slider design and modeling.